In the production of films, in particular stretched very thin and thin films with thicknesses between 1 .mu.m and 30 .mu.m, plastic granules are first melted in an extruder, and the extruded plastic melt is fed to a slot die. The prefilm emerging from this slot die is laid onto a take-off and chill roll and, after leaving the take-off roll, is stretched in and transverse to the machine-running direction in a stretching frame.
In order to stabilize the laying of the prefilm onto the take-off roll, in the prior art additional forces are applied to the prefilm in the region between the emergence of the prefilm from the slot die and the point at which it is laid on the take-off roll. The additional forces applied can be produced using an air jet from an air knife to blow against the prefilm, the spraying of water into the gore of the prefilm before the point at which it is laid on the take-off roll, the formation of a water film on the take-off roll and the generation of an electric field by means of an electrode at high voltage, the field lines of the electric field being directed toward the surface of the take-off roll. The forces additionally applied are used for the lateral outward displacement of air enclosed between the surface of the take-off roll and the underside of the prefilm, that is the side of the prefilm facing the take-off roll, under the gore of the prefilm before the prefilm is laid onto the take-off roll, in order in this way to stabilize the laying of the prefilm onto the roll surface over a predefined wrap angle. If this is not successful, air inclusions that are more or less large, so-called "pinning" bubbles are produced, which are located between the underside of the prefilm and the surface of the take-off roll, and impair the smoothness and thus the quality of the prefilm cooled down on the take-off roll, as well as the film biaxially stretched therefrom.
It is known to arrange a metal strip made of stainless steel parallel to the take-off roll surface as electrode, the metal strip generally being stamped out or cut out from a metal foil of about 20 .mu.m thickness in a width of 10 mm. A metal strip of this type has the disadvantage that it cannot absorb sufficiently high clamping forces, which are necessary to clamp the metal strip with the desired accuracy at a uniform distance from the take-off roll. Furthermore, it is disadvantageous that the edges of the metal strip produced by the cutting or stamping process are irregular, with the result that the electric field between the metal strip and the take-off roll is distorted. These distortions of the electric field are also further amplified on account of an irregular spacing of the metal strip from the take-off roll surface. The distortions of an electric field lead on the one hand to electric flashovers, as a result of which the prefilm can be damaged, and on the other hand to a nonuniform laying of the prefilm onto the take-off roll, as a result of which air inclusion between the prefilm and the take-off roll can continue to occur. The distortions of the electric field overall do not lead to the desired film quality.
The PCT Application WO 95/21735 (German Utility Model G 94 02 027) discloses a device for the production of films, in particular thin or very thin films, having a rotating roll, using which a plastically deformable film, preferably supplied through a slot die, is conveyed and oriented, an electric voltage, through which the supplied film is laid onto the roll, being applied between the roll and a metal strip that is arranged parallel to and at a short distance from the roll; in this device the metal strip has a sharp longitudinal edge which is arranged lying next to the roll and is directed toward the latter. The sharp longitudinal edge is constructed in the manner of a cutting edge on a sharply tapering region of the metal strip, the metal strip being arranged lying next to the roll at a distance smaller than 6 mm. The center line of the sharply tapering region is aligned with the center of the roll. The use of such a metal-strip electrode improves the film quality, but not to the desired extent, since although the air inclusions become smaller in their diameters and fewer in their number, this is in no way to the extent which is sought.
U.S. Pat. No. 5,494,619 discloses an electrostatic pinning process in which a wire electrode is arranged between an extrusion die and a take-off roll. A shield at bias voltage is located between the extrusion die and the wire electrode. The bias voltage of the shield is 1 to 10 kV, whereas a voltage of 6 to 15 kV is applied to the wire electrode. In the case of this process, it is essential to apply the highest possible electrostatic charge to the film to be laid onto the take-off roll, in comparison with an arrangement in which the shielding is at ground potential. To this end, there is an optimum bias voltage of the shield, which depends on the voltage of the wire electrode, but differs significantly from the latter and from ground potential. The net current or the electrostatic charge to the film results from the difference between the current from the wire electrode and a previously measured current to the shield, which is obtained by applying voltage to the shield and the wire electrode being at ground potential. For a predefined voltage, which is applied to the wire electrode, a high current generation results from a low electric voltage on the shielding, but also a great number of field lines, the majority of which end on the shield and not on the film. By increasing the voltage on the shield, the current decreases, but more field lines end on the film, as a result of which the major part of the current is deflected onto the film. The overlaying of two electrostatic fields does not take place, since the electrically biased shield serves to deflect field lines directed toward it from the wire electrode in the direction of the film.
The object of the invention is to develop further and to improve a device of the type described at the beginning in such a way that the air inclusions become significantly smaller and fewer than in the case of laying devices for films known in the prior art.